1. Field of the Invention
The invention relates to blocked polyisocyanates, their preparation and use in one-component coating materials.
2. Description of the Related Art
The blocking of polyisocyanates for the temporary protection of the isocyanate groups is a known working method and is described in, for example, Houben Weyl, Methoden der organischen Chemie XIV/2, pp. 61-70. Curable compositions comprising blocked polyisocyanates find use in, for example, polyurethane (PU) coating materials.
The preparation of one-component, storage-stable binders for PU baking varnishes by blending blocked polyisocyanates with OH-containing polycondensates or addition polymers (polyesters or polyacrylates) is known.
In these one-component coating materials the blocking agent fulfils two functions: firstly, it prevents premature reaction of the OH component with the NCO groups it blocks, and secondly, by virtue of its specific deblocking attributes, it regulates the curing of the coating materials in a defined temperature range. As well as these desired properties, however, the individual blocking agents also bring with them unwanted properties, such as crystallization or yellowing tendency, deficient economics, environmental problems and critical physiological effects, for example.
This can be illustrated using butanone oxime and 3,5-dimethylpyrazole as examples. Both blocking agents are readily compatible with the known paint polyisocyanates and deblock within about 30 minutes at 130-140° C. On the minus side, butanone oxime leads to thermal yellowing of the baked coating, and the substance per se is toxicologically objectionable. Dimethylpyrazole is relatively expensive to prepare from acetylacetone and hydrazine hydrate, and gives coatings an unpleasant odour (see, for example, T. Engbert, E. König, E. Jürgens, Farbe & Lack, Curt R. Vincentz Verlag, Hanover 97/1996).
The elimination of the blocking agent and its gaseous state from the coating film may lead, furthermore, to blistering in the coating. Incineration of the emitted blocking agent may be necessary where appropriate. An overview of blocking agents suitable in principle is found, for example, in Wicks et al. in Progress in Organic Coatings 1975, 3, 73-79, 1981, 9, pp. 3-28 and 1999, 36, pp. 148-172.
For the coil coatings field, the blocked polyisocyanates must be crosslinkable within a very short time at baking temperatures of up to 254° C. PMT (peak metal temperature) and during the baking operation the polymers must exhibit very little thermal yellowing, preferably none. The baking temperature required is dependent primarily on the reactivity of the blocked polyisocyanate and/or on the catalyst used for this process.